Volcanic Cooling

The increasing concentrations of greenhouse gases in the atmosphere
are not the only factors influencing climate. Explosive volcanic
eruptions can inject enormous amounts of sulfur dioxide and ash
into the atmosphere. Aerosol particles injected into the stratosphere
can result in climate changes lasting up to several years. Observed
climatic responses to the Mt. Pinatubo eruption have included
tropospheric cooling, stratospheric warming, and an overall drop of
about 0.5°C in the global average surface temperature.

There are not yet comprehensive estimates of how the effects of
changes in aerosol concentrations, changes in land cover and land
use, and changes in concentrations of greenhouse gases will combine
with natural influences to alter the global climate. Examination of
the temperature record of the last 100 years does show a warming of
about 0.5°C, only temporarily reversed recently by the
volcanic influence of Mt. Pinatubo, suggesting that the enhanced
greenhouse effect is exerting the primary influence. The fact that
this warming is somewhat less and different in timing than that
predicted by computer models emphasizes the need for continuing
research directed toward gaining a better understanding of both
human and natural influences such as solar variability on the climate
system.